Resistance and hypersensitivity to thyroid hormone (TH) are inherited defects. The former is associated with mutations in the TH receptor beta gene, and often results in growth retardation and mental disabilities. The cause of the latter is unknown. The aim of this project is to assess tissue responses to L-T3 in affected individuals, unaffected relatives, and unrelated controls in order to better define these syndromes and standardize diagnostic procedures.